This invention relates to the preparation of polycarbonates by the melt reaction of a bisphenol with an ester-substituted diaryl carbonate. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products and possessing a high level of endcapping.
Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride , an acid acceptor such as sodium hydroxide and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.
The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.
The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer""s flow properties and performance.
Some years ago, it was reported in U.S. Pat. No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterification catalyst in a the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration.
It would be desirable, therefore, to minimize the amount of catalyst required in the for the melt preparation of polycarbonate from bisphenols and ester substituted diaryl carbonates such as bis-methyl salicyl carbonate.
The present invention provides a method for preparing polycarbonate comprising heating a mixture comprising a catalyst; at least one diaryl carbonate having structure I 
wherein R1 and R2 are independently C1-C20 alkyl radicals, C4-C20 cycloalkyl radicals, or C4-C20 aromatic radicals, R3 and R4 are independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, C4-C20 aromatic radical, C1-C20 alkoxy radical, C4-C20 cycloalkoxy radical, C4-C20 aryloxy radical, C1-C20 alkylthio radical, C4-C20 cycloalkylthio radical, C4-C20 arylthio radical, C1-C20 alkylsulfinyl radical, C4-C20 cycloalkylsulfinyl radical, C4-C20 arylsulfinyl radical, C1-C20 alkylsulfonyl radical, C4-C20 cycloalkylsulfonyl radical, C4-C20 arylsulfonyl radical, C1-C20 alkoxycarbonyl radical, C4-C20 cycloalkoxycarbonyl radical, C4-C20 aryloxycarbonyl radical, C2-C60 alkylamino radical, C6-C60 cycloalkylamino radical, C5-C60 arylamino radical, C1-C40 alkylaminocarbonyl radical, C4-C40 cycloalkylaminocarbonyl radical, C4-C40 arylaminocarbonyl radical, or C1-C20 acylamino radical; and b and c are independently integers 0-4; and at least one dihydroxy aromatic compound, said catalyst comprising at least one source of alkaline earth ions or alkali metal ions, and at least one quaternary ammonium compound, quaternary phosphonium compound, or a mixture thereof, said source of alkaline earth ions or alkali metal ions being present in an amount such that between about 10xe2x88x925 and about 10xe2x88x928 moles of alkaline earth metal ions or alkali metal ions are present in the mixture relative per mole of dihydroxy aromatic compound employed, said quaternary ammonium compound, quaternary phosphonium compound or mixture thereof being present in an amount between about 2.5xc3x9710xe2x88x923 and about 1xc3x9710xe2x88x926 moles per mole of dihydroxy aromatic compound employed.
The present invention further relates to a method for forming polycarbonates by reaction of an ester-substituted diaryl carbonate in which the level of Fries rearrangement product in the product polycarbonate is less than about 1000 parts per million (ppm) and the level of internal ester carbonate linkages in the product polycarbonate is less than about 1 percent of the total number of moles of dihydroxy aromatic compound employed and the level of terminal hydroxy ester groups in the product polycarbonate is less than about 1 percent of the total number of moles of dihydroxy aromatic compound employed.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy aromatic compound.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.
xe2x80x9cCatalyst systemxe2x80x9d as used herein refers to the catalyst or catalysts that catalyze the transesterification of the bisphenol with the diaryl carbonate in the melt process.
The terms xe2x80x9cbisphenolxe2x80x9d, xe2x80x9cdiphenolxe2x80x9d and xe2x80x9cdihydric phenolxe2x80x9d as used herein are synonymous.
xe2x80x9cCatalytically effective amountxe2x80x9d refers to the amount of the catalyst at which catalytic performance is exhibited.
As used herein the term xe2x80x9cFries productxe2x80x9d is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, among the Fries products within the product polycarbonate are those structural units, for example structure VIII below, which afford 2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate.
The terms xe2x80x9cFries productxe2x80x9d and xe2x80x9cFries groupxe2x80x9d are used interchangeably herein.
The terms xe2x80x9cFries reactionxe2x80x9d and xe2x80x9cFries rearrangementxe2x80x9d are used interchangeably herein.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclopropyl, cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.
In the present invention it has been discovered that extremely low levels of catalyst may be employed to prepare polycarbonate using the melt reaction of an ester substituted diaryl carbonate with a bisphenol. The use of very low catalyst loadings is desirable from at least two perspectives. First, the use of low catalyst levels during melt polymerization tends to suppress the formation of undesired Fries rearrangement products. Second, because residual catalyst present in the polymer tends to decrease the useful life span of articles made from it by increasing water absorption, decreasing thermal stability and promoting discoloration, its minimization is desirable. The polycarbonate prepared by the method of the present invention is free of, or contains undetectable levels of Fries rearrangement products. Moreover, in the absence of an added exogenous monofunctional phenol the product polycarbonate is very highly endcapped with less than 50% of the endgroups being free hydroxyl groups. Where an exogenous monofunctional phenol is added to the polymerization mixture, high levels of incorporation of said phenol are observed. In this manner both the identity of the polymer endgroups and the polymer molecular weight may be controlled in the melt reaction.
In the process of the present invention an ester-substituted diaryl carbonate having structure I is reacted under melt reaction conditions with at least one dihydroxy aromatic compound in the presence of at least one source of alkaline earth ions or alkali metal ions, and an organic ammonium compound or an organic phosphonium compound or a combination thereof. Ester-substituted diaryl carbonates I are exemplified by bis-methyl salicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicyl carbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate, bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate and the like. Typically bis-methyl salicyl carbonate is preferred.
The dihydroxy aromatic compounds of the present invention are selected from the group consisting of bisphenols having structure II, 
wherein R5-R12 are independently a hydrogen atom, halogen atom, nitro group, cyano group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C6-C20 aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO2 group, a C6-C20 aromatic radical, a C6-C20 cycloaliphatic radical, or the group 
wherein R13 and R14 are independently a hydrogen atom, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C4-C20 aryl radical; or R13 and R14 together form a C4-C20 cycloaliphatic ring which is optionally substituted by one or more C1-C20 alkyl, C6-C20 aryl, C5-C21 aralkyl, C5-C20 cycloalkyl groups, or a combination thereof; dihydroxy benzenes having structure III 
wherein R15 is independently at each occurrence a hydrogen atom, halogen atom, nitro group, cyano group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C4-C20 aryl radical, d is an integer from 0 to 4; and
dihydroxy naphthalenes having structures IV and V 
wherein R16, R17, R18 and R19 are independently at each occurrence a hydrogen atom, halogen atom, nitro group, cyano group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C4-C20 aryl radical; e and f are integers from 0 to 3, g is an integer from 0 to 4, and h is an integer from 0 to 2.
Suitable bisphenols II are illustrated by 2,2-bis(4-hydroxyphenyl)propane (bisphenol A); 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 2,2-bis(3-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-dichloro-4-hydroxyphenyl)-propane; 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-disopropyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-di-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-diphenyl-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)propane; 2,2-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)cyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; bis(3-chloro-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 4,4xe2x80x2dihydroxy-1,1-biphenyl; 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dimethyl-1,1-biphenyl; 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dioctyl-1,1-biphenyl; 4,4xe2x80x2-dihydroxydiphenylether; 4,4xe2x80x2-dihydroxydiphenylthioether; 1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene; 1,3-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene; 1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene and 1,4-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene.
Suitable dihydroxy benzenes III are illustrated by hydroquinone, resorcinol, methylhydroquinone, phenylhydroquinone, 4-phenylresorcinol and 4-methylresorcinol.
Suitable dihydroxy naphthalenes IV are illustrated by 2,6-dihydroxy naphthalene; 2,6-dihydroxy-3-methyl naphthalene; and 2,6-dihydroxy-3-phenyl naphthalene.
Suitable dihydroxy naphthalenes V are illustrated by 1,4-dihydroxy naphthalene; 1,4-dihydroxy-2-methyl naphthalene; 1,4-dihydroxy-2-phenyl naphthalene and 1,3-dihydroxy naphthalene.
The catalyst used in the method of the present invention comprises at least one source of alkaline earth ions or alkali metal ions, and at least one quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof, said source of alkaline earth ions or alkali metal ions being used in an amount such that the amount of alkaline earth or alkali metal ions present in the reaction mixture is in a range between about 10xe2x88x925 and about 10xe2x88x928 moles alkaline earth or alkali metal ion per mole of dihydroxy aromatic compound employed.
The quaternary ammonium compound is selected from the group of organic ammonium compounds having structure VI 
wherein R20-R23 are independently a C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or a C4-C20 aryl radical; and Xxe2x88x92 is an organic or inorganic anion. In one embodiment of the present invention anion Xxe2x88x92 is selected from the group consisting of hydroxide, halide, carboxylate, sulfonate, sulfate, formate, carbonate, and bicarbonate.
Suitable organic ammonium compounds comprising structure VI are illustrated by tetramethyl ammonium hydroxide, tetrabutyl ammonium hydroxide, tetramethyl ammonium acetate, tetramethyl ammonium formate and tetrabutyl ammonium acetate.
The quaternary phosphonium compound is selected from the group of organic phosphonium compounds having structure VII 
wherein R24-R27 are independently a C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or a C4-C20 aryl radical; and Xxe2x88x92 is an organic or inorganic anion. In one embodiment of the present invention anion Xxe2x88x92 is an anion selected from the group consisting of hydroxide, halide, carboxylate, sulfonate, sulfate, formate, carbonate, and bicarbonate. Suitable organic phosphonium compounds comprising structure VII are illustrated by tetramethyl phosphonium hydroxide, tetramethyl phosphonium acetate, tetramethyl phosphonium formate, tetrabutyl phosphonium hydroxide, and tetrabutyl phosphonium acetate.
Where Xxe2x88x92 is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structures VI and VII are properly balanced. For example, where R20-R23 in structure VI are each methyl groups and Xxe2x88x92 is carbonate, it is understood that Xxe2x88x92 represents xc2xd (CO3xe2x88x922).
Suitable sources of alkaline earth ions include alkaline earth hydroxides such as magnesium hydroxide and calcium hydroxide. Suitable sources of alkali metal ions include the alkali metal hydroxides illustrated by lithium hydroxide, sodium hydroxide and potassium hydroxide. Other sources of alkaline earth and alkali metal ions include salts of carboxylic acids, such as sodium acetate and derivatives of ethylene diamine tetraacetic acid (EDTA) such as EDTA tetrasodium salt, and EDTA magnesium disodium salt.
In the method of the present invention an ester-substituted diaryl carbonate I, at least one dihydroxy aromatic compound and a catalyst are contacted in a reactor suitable for conducting melt polymerization. The relative amounts of ester-substituted diaryl carbonate and dihydroxy aromatic compound are such that the molar ratio of carbonate I to dihydroxy aromatic compound is in a range between about 1.20 and about 0.8, preferably between about 1.10 and about 0.9 and still more preferably between about 1.05 and about 1.01.
The amount of catalyst employed is such that the amount of alkaline earth metal ion or alkali metal ions present in the reaction mixture is in a range between about 1xc3x9710xe2x88x925 and about 1xc3x9710xe2x88x928, preferably between about 5xc3x9710xe2x88x925 and about 1xc3x9710xe2x88x927, and still more preferably between about 5xc3x9710xe2x88x925 and about 5xc3x9710xe2x88x927 moles of alkaline earth metal ion or alkali metal ion per mole dihydroxy aromatic compound employed. The quaternary ammonium compound, quaternary phosphonium compound or a mixture thereof is used in an amount corresponding to about 2.5xc3x9710xe2x88x923 and 1xc3x9710xe2x88x926 moles per mole dihydroxy aromatic compound employed.
Typically the ester-substituted diaryl carbonate, at least one dihydroxy aromatic compound and the catalyst are combined in a reactor which has been treated to remove adventitious contaminants capable of catalyzing both the transesterification and Fries reactions observed in uncontrolled melt polymerizations of diaryl carbonates with dihydroxy aromatic compounds. Contaminants such as sodium ion adhering to the walls of a glass lined reactor are typical and may be removed by soaking the reactor in mild acid, for example 3 normal hydrochloric acid, followed by removal of the acid and soaking the reactor in high purity water, such as deionized water.
In one embodiment of the present invention an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, at least one dihydroxy aromatic compound, such as BPA, and a catalyst comprising alkali metal ions, such as sodium hydroxide, and a quaternary ammonium compound, such as tetramethyl ammonium hydroxide, or a quaternary phosphonium compound, such as tetrabutyl phosphonium acetate, are charged to a reactor and the reactor is purged with an inert gas such as nitrogen or helium. The reactor is then heated to a temperature in a range between about 100xc2x0 C. and about 340xc2x0 C., preferably between about 100xc2x0 C. and about 280xc2x0 C., and still more preferably between about 140xc2x0 C. and about 240xc2x0 C. for a period of from about 0.25 to about 5 hours, preferably from about 0.25 to about 2 hours, and still more preferably from about 0.25 hours to about 1.25 hours. While the reaction mixture is heated the pressure over the reaction mixture is gradually reduced from ambient pressure to a final pressure in a range between about 0.001 mmHg and about 400 mmHg, preferably 0.01 mmHg and about 100 mmHg, and still more preferably about 0.1 mmHg and about 10 mmHg.
Control of the pressure over the reaction mixture allows the orderly removal of the phenolic by-product formed when the dihydroxy aromatic compound undergoes a transesterification reaction with a species capable of releasing a phenolic by-product, for example bis-methyl salicyl carbonate or a growing polymer chain endcapped by a methyl salicyl group. As noted above the reaction may be conducted at subambient pressure. In an alternate embodiment of the present invention the reaction may be conducted at slightly elevated pressure, for example a pressure in a range between about 1 and about 2 atmospheres.
As noted, the use of excessive amounts of catalyst may affect negatively the structure and properties of a polycarbonate prepared under melt polymerization conditions. The present invention provides a method of melt polymerization employing a highly effective catalyst system comprising at least one source of alkaline earth or alkali metal ions, and a quaternary ammonium compound or quaternary phosphonium compound or mixture thereof which provides useful reaction rates at very low catalyst concentrations, thereby minimizing the amount of residual catalyst remaining in the product polycarbonate. Limiting the amount of catalyst employed according to the method of the present invention provides a new and useful means of controlling the structural integrity of the product polycarbonate as well. Thus, the method of the present invention provides a product polycarbonate having a weight average molecular weight, as determined by gel permeation chromatography, in a range between about 10,000 and about 100,000 Daltons, preferably between about 15,000 and about 60,000 Daltons, and still more preferably between about 15,000 and about 50,000 Daltons said product polycarbonate having less than about 1000, preferably less than about 500, and still more preferably less than about 100 parts per million (ppm) Fries product. Structure VIII below 
illustrates the Fries product structure present in a polycarbonate prepared from bisphenol A. As indicated, the Fries product may serve as a site for polymer branching, the wavy lines indicating polymer chain structure.
In addition to providing a product polycarbonate containing only very low levels of Fries products, the method of the present invention provides polycarbonates containing very low levels of other undesirable structural features which arise from side reactions taking place during melt the polymerization reaction between ester-substituted diaryl carbonates I and dihydroxy aromatic compounds. One such undesirable structural feature has structure IX 
and is termed an internal ester-carbonate linkage. Structure IX is thought to arise by reaction of an ester-substituted phenol by-product, for example methyl salicylate, at its ester carbonyl group with a dihydroxy aromatic compound or a hydroxy group of a growing polymer chain. Further reaction of the ester-substituted phenolic hydroxy group leads to formation of a carbonate linkage. Thus, the ester-substituted phenol by-product of reaction of an ester-substituted diaryl carbonate with a dihydroxy aromatic compound, may be incorporated into the main chain of a linear polycarbonate. The presence of uncontrolled amounts of ester carbonate linkages in the polycarbonate polymer chain is undesirable.
Another undesirable structural feature present in melt polymerization reactions between ester-substituted diaryl carbonates and dihydroxy aromatic compounds is the ester-linked terminal group having structure X 
which possesses a free hydroxyl group. Structure X is thought to arise in the same manner as structure IX but without further reaction of the ester-substituted phenolic hydroxy group. The presence of uncontrolled amounts of hydroxy terminated groups such as X is undesirable. In structures VIII, IX and X the wavy line shown as 
represents the product polycarbonate polymer chain structure.
The present invention, in sharp contrast to known methods of effecting the melt polymerization of an ester-substituted diaryl carbonate and a dihydroxy aromatic compound, provides a means of limiting the formation of internal ester-carbonate linkages having structure IX as well as ester-linked terminal groups having structure X, during melt polymerization. Thus in a product polycarbonate prepared using the method of the present invention structures, IX and X, when present, represents less than 1 mole percent of the total amount of all structural units present in the product polymer derived from dihydroxy aromatic compounds employed as starting materials for the polymer synthesis.
An additional advantage of the method of the present invention over earlier methods of melt polymerization of ester-substituted diaryl carbonates and dihydroxy aromatic compounds, derives from the fact that the product polymer is endcapped with ester-substituted phenoxy endgroups and contains very low levels, less than about 50 percent, preferably less than about 10 percent, and still more preferably less than about 1 percent, of polymer chain ends bearing free hydroxy groups. The ester substituted terminal groups are sufficiently reactive to allow their displacement by other phenols such as p-cumylphenol. Thus, following the melt polymerization the product polycarbonate may be treated with one or more exogenous phenols to afford a polycarbonate incorporating endgroups derived from the exogenous phenol. The reaction of the ester substituted terminal groups with the exogenous phenol may be carried out in a first formed polymer melt or in a separate step.
In one embodiment of the present invention an exogenous monofunctional phenol, for example p-cumylphenol, is added at the outset of the reaction between the ester substituted diaryl carbonate and the dihydroxy aromatic compound. The product polycarbonate then contains endgroups derived from the exogenous monofunctional phenol. The exogenous monofunctional phenol serves both to control the molecular weight of the product polycarbonate and to determine the identity of the polymer endgroups. The exogenous monofunctional phenol may be added in amounts ranging from about 0.1 to about 10 mole percent, preferably from about 0.5 to about 8 mole percent and still more preferably from about 1 to about 6 mole percent based on the total number of moles of dihydroxyaromatic compound employed in the polymerization. Additional catalyst is not required apart from the catalytically effective amount added to effect the polymerization reaction. Suitable exogenous monofunctional phenols are exemplified by p-cumylphenol; 2,6-xylenol; 4-t-butylphenol; p-cresol; 1-naphthol; 2-naphthol; cardanol; 3,5-di-t-butylphenol, p-nonylphenol; p-octadecylphenol; and phenol. In alternative embodiments of the present invention the exogenous monofunctional phenol may be added at an intermediate stage of the polymerization or after its completion. In such alternative embodiments the exogenous phenol may exert a controlling effect upon the molecular weight of the product polycarbonate and will control the identity of the polymer terminal groups.
The present invention may be used to prepare polycarbonate products having very low levels (less than 1 ppm) of trace contaminants such as iron, chloride ion, and sodium ion. Where such extremely low levels of trace contaminants is desired it is sufficient to practice the invention using starting materials, ester-substituted diary carbonate and dihydroxy aromatic compound having correspondingly low levels of the trace contaminants in question. For example, the preparation bisphenol A polycarbonate containing less than 1 ppm each of iron, chloride ion and sodium ion may be made by the method of the present invention using starting materials bis-methyl salicyl carbonate and bisphenol A containing less than 1 ppm iron, chloride ion and sodium ion.
The method of the present invention can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring and the presence of adventitious catalysts can be controlled. It is preferable that the reactor is capable of stirring in high viscosity conditions as the viscosity of the reaction system is increased in later stages of the reaction.
Polycarbonates prepared using the method of the present invention may be blended with conventional additives such as heat stabilizers, mold release agents and UV stabilizers and molded into various molded articles such as optical disks, optical lenses, automobile lamp components and the like. Further, the polycarbonates prepared using the method of the present invention may be blended with other polymeric materials, for example, other polycarbonates, polyestercarbonates, polyesters and olefin polymers such as ABS.